Cruachan Power Station
Conceived in the 1930s and built in the 1960s, this is the world's first pumped-storage hydroelectric plant. At night the electricity grid provides free (surplus) power to Cruachan to pump water from Loch Awe up 1,300 ft to the reservoir. Then during the day the turbines reverse and Cruachan can provide power back to the grid at 28 seconds notice, to manage demand spikes. The power station actually uses more electricity than it generates, but the economic key is in the timing. Rainfall provides only about 10-15% of the reservoir's water.
There are three more power stations like this in the UK, and hundreds more around the world.
As an aside, we learned that any Scottish hill over 3,000ft high is called a "Munro". Ben Cruachan is 3,600ft; the highest Munro in Argyll and Bute.
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